This is a new field. I tried deleting and creating it as a single-line text field. I then called the contact property API to update the field to datetime. Even though the update API returns success, if I view the field in HubSpot, I can still see the error. Attached is the screen shot before and after update

@jmichaelRS Did this have the same internal name as a previous field? Not sure if that would be the issue or not. I also wonder if it conflicts with an internal name that is used with our SF integration. Are you currently using the SF integration from HubSpot?